Hand-held vacuum and pressure pumps are generally useful whenever vacuum or pressure is desired. Vacuum or pressure can be created, for example, by compressing (i.e. squeezing) and releasing a handle of such a vacuum or pressure pump. Generally, such squeezing and releasing causes a piston to move in a cylinder of the pump thereby creating vacuum or pressure. Many types of vacuum pumps have been devised, but they often suffer from such drawbacks as complexity, expense, excessive bulk, inability to pull a suitable vacuum, and the like. The vacuum pump of the referenced patents has significantly solved the need for a vacuum pump which is simple, inexpensive, lightweight, compact, and portable, and one which can pull a useful vacuum.
Such hand-held vacuum and pressure pumps are especially useful for various tasks such as aiding in performing vacuum extractions during childbirth, and are useful in various industries, such as the automotive industry, for liquid sampling and vacuum system testing and repair. Vacuum pumps manufactured according to the aforesaid patents have the ability to pull a vacuum of, for example, twenty-eight inches of mercury.
In some applications, it is desirable to pull a preset or controlled vacuum, and one which is repeatable. Inasmuch as the hand-held vacuum pump is manually operated by hand and because the pump can quickly pull a relatively high vacuum, it is difficult to manually pull a given level of vacuum. One way to accomplish this is disclosed in U.S. Pat. No. 4,979,883. That patent discloses a vacuum limiter that uses two distinct valves, one to meter the vacuum pressure, and the second to hold the vacuum. Both of these valves are located on the source side of the vacuum pump, so the valves have to maintain the difference in pressure between the vacuum and the vacuum cylinder. This leads to problems with drawing an accurate vacuum and holding it.